Monday, February 1, 2010

Multiple Flashes With a Little Physics on the Side

This was fun. Courtney is a photography student and when she came to the studio for her portraits I wanted to do something that would blow her mind. So we left the nice part of my studio and set up a few speed-lights in the long dark hall that leads to the rear exit. We are still in the process or remodeling so this hall is what some might call "industrial".

The camera was placed on a tripod and the aperture set to the maximum for my lens so I could get the most depth of field. The shutter was set to 30 seconds. Then came the hard part. Courtney and I had to work in the complete dark. My job was to fire the strobe with the test button from different angles while keeping it out of the frame so I didn't' show up in the image and she had to navigate around the room in the dark. The 30 second exposure felt like for ever.

It took several tries to get the result but here's the final product. No photoshop necessary. This is all done within one frame. This technique can work for still or moving objects as long as the flash is your only light source, it must be completely dark.

In college we did this same technique to get greater depth of field in the studio for product shots. For example, say I was shooting a photo of a large product, like an SUV in studio (never-mind the logistics of getting it in the studio) If I didn't have enough power to get the maximum aperture with one flash, I could increase the number of flashes to increase the exposure.

Now you could do this as a guess and check but there is also an easy equation that will save you time.
E=l/d is the mathematical representation of the way light works according to the inverse square law.  (The link has a good explanation of this.) In simple terms this expresses the relationship between the distance (d) from the light source and the required intensity (l) for correct exposure (e).

To increase depth of field by adding additional flashes we solve for "l" to know the correct number of times the flash most go off for proper exposure at our chosen aperture. The law says that an object 2x the distance from a light source will receive a 1/4 of the illumination. By moving your subject from 4 feet away to 8 feet away, you will need four times the amount of light for the same exposure.

For example: If Courtney is four feet away from the speedlight at full power and I want to shoot at F22 but my current meter reading is saying I have enough power for F8 I'll need to use the flash four times in the same shot to get the correct exposure and make up the two stop difference.

What you may not realize is that this law is in action no matter the light source you're using. Have you ever thought to yourself, "oh my subject just moved farther away from me, I need to open up to a larger aperture?" Then you've just used the inverse square law without even thinking about it. Pretty cool yes?

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